1. Field of the Invention
This invention relates to furnaces for heating glass sheets and more particularly to a system and method for preventing the arching of sheet glass, and particularly glass sheets having a low emissivity coating, in a horizontal tempering conductive heat furnace.
2. Description of the Prior Art
It is known that patents exist whereby plate glass, as it passes along rollers within a furnace, can be heating using radiative (e.g. heated coils operating on the same principal as a toaster), conductive (e.g. contact with a heated surface such as rollers), or convective (e.g. hot air blown on the glass). Examples of furnaces using these various heating mechanisms are shown in U.S. Pat. No. 4,505,671 (McMaster). Other known patents are U.S. Pat. No. 3,326,654 to Plumat, U.S. Pat. No. 3,488,178 to Welker and U.S. Pat. No. 3,402,038 to Hordis. These patents appear to only disclose convective heating whereby air heated within the confines of the furnace is circulated over the glass via compressed air nozzles.
Annealed glass sheets are processed to tempering grades in furnaces utilizing radiation heat transfer as the primary energy source. Typically heating coils comprised of serpentine or helical nichrome wire are arranged in a spatial relationship with the glass surfaces such that the sheet is uniformly heated to high temperatures approaching 615° C. (1139° F.) and then air quenched in subsequent processing.
Manufacturers of glass tempering furnaces have favored radiative designs. These designs are field proven with a minimum of internal components and complexity, and process glass to uniform material and optical qualities. Standard float glass exhibits emissivity values of 0.85 and higher, leading to production times and material qualities in radiative furnaces acceptable to processing plants.
Architectural styles and building codes have changed, however, thus introducing increasing surface areas suitable for tempered glass such as doors, windows, and exterior glazing in both residential and commercial structures. Glass manufacturers are sensitive to the increasing energy requirements of the building industry. One priority is to reduce the solar load transmitted through this glass exterior. To minimize the solar influx, the exterior exposed face of the glass has been modified by application of sputtered reflecting films, etchings, or surface treatments such that the solar infrared radiation incident on the surface is highly reflected and the visible light spectrum transmitted. The interior glass face, not requiring specialty treatments, retains the emissivity and heating characteristics of typical annealed glass. Glass treated in this manner is marketed as low-E, or low-emissivity glass. Emissivities are generally stated as 0.15 and lower, with special treatments capable of producing emissivity values as low as 0.04. Though usage of this energy efficient glass is popular for the discussed energy reasons, conventional tempering using radiation heating means is quite difficult.
In a standard radiant furnace, when the high-performance Low-E glass in conveyed into the furnace, the bottom skin of the glass, which does not have the coating, receives its heat at the normal rate from the conduction of the ceramic rolls. The top skin however reflects most of the radiant energy being produced by the heating elements and does not absorb much heat. This causes the bottom skin to expand much more than the top skin and causes the glass to bow or dish up, inside the furnace. This phenomenon occurs on normal uncoated glass also but it is a very short-lived condition. In other words the top will absorb heat at a rate that will allow the skin temperatures to equalize. When this bowing occurs, there are several problems that are caused. One of the problems is related to high heat transfer from the ceramic rolls to the bottom surface of the glass due to the weight of the glass being concentrated in a smaller contact area. One of the most severe problems is that while in this bowed state, the glass is no longer contacting the ceramic rolls except for the reduced area in the middle and is no longer receiving heating from conduction. This will lead to very non-uniform heating of the glass and will result in breakage, warpage, or exceedingly long heating times.
Accordingly, a need remains for an improved glass sheet heating furnace that overcomes the drawbacks in the prior art.